This invention relates to a current sensor with a temperature compensation capability intended for use in an electrical apparatus such as a residual current correction device, a current meter or a power meter.
It is an aim of the present invention to provide a current sensor in economical form which includes temperature sensing means for facilitating temperature compensation.
In accordance with the invention there is provided a current sensor comprising a metallic link member having two end portions of conductive material and an intermediate portion interconnecting the end portions, said intermediate portion being formed of a resistive material, and an integrated circuit analog to digital converter mounted on said link member, said converter having analog input terminals connected to respective ones of said two end portions and digital output terminals for connection to a processing apparatus, wherein a temperature sensor is provided on or within said intermediate portion.
The temperature sensor is preferably an electronic semiconductor temperature sensor and may be mounted directly onto the intermediate portion with a thermally suitable conducting glue. The temperature sensor may be built into (i.e. integrated) into the integrated circuit analog to digital converter in which case it will form part of a semiconductor die mounted directly onto the intermediate portion.
Embodiments of the invention have the advantage that the temperature sensor will follow the temperature of the intermediate portion very closely. It is therefore possible to compensate for changes in the resistance of the intermediate portion resulting from temperature variations as the current flowing therethrough changes.
Conveniently, the converter is attached to the intermediate portion by a layer of electrically insulative adhesive material and the analog input terminals of the converter are connected to the end portions by wire bonds.
The converter preferably includes a delta-sigma modulator which provides a high frequency one-bit digital data. One or more decimation filtering stages may be included in the converter.
The converter may also have a voltage reference terminal for connection to a reference voltage source, the converter operating to provide digital output signals respectively representing the current flowing through said intermediate portion and digital output signals representing the voltage on one of said end portions.
Embodiments of the invention may be advantageously employed in residual current devices. Conventionally, residual current is detected utilising a current transformer having primary windings through which, in the case of a single phase device, load current flows in opposite directions so that if the return current is different from the outwardly flowing current because of current leakage an output current signal is induced in a secondary winding of the transformer. In the case of a multi-phase device, primary windings of the transformer are connected in all of the phase lines and the neutral line. In normal situations, when there is no current leakage, the net current induced in the secondary winding is zero and therefore no output is detected.
Sophisticated materials have been developed for the core of the current transformer, which enable considerable accuracy to be obtained when the currents flowing in the primary windings are substantially sinusoidal. However, switch mode power supplies are often used for computers and other equipment and there is an increasing tendency for such equipment to cause dc offsets in the currents. Such developments have made detectors utilising current transformers less reliable and prone to false tripping or failure to detect a dc current leakage.
This is a particular problem in the case of directly actuated electro-mechanical devices, where the current transformer secondary winding actually drives an actuator. The situation is not much improved, when including an electronic detection and amplification means connected to the secondary winding, as there are still problems with high frequency transients and dc offsets. A very small dc current level can cause the core to saturate thereby seriously impairing the ability of the detector to detect current leakage.
It is also an aim of the present invention to provide a residual current detection device in which the above mentioned problems are substantially overcome in a simple and efficacious manner.
In accordance with the invention there is further provided a residual current detection device comprising a plurality of resistive shunts for connection in respective ones of a plurality of lines though which current can flow to and from a load, and detector means sensitive tothe voltage developed across each of the shunts to detect any imbalance between the currents flowing though the shunts, wherein a temperature compensation means is provided for facilitating compensation for fluctuations in shunt resistance with variations in temperature.
In preferred embodiments, the temperature compensation means is a temperature sensor provided on or within each of said plurality of resistive shunts.
Preferably, the detector means comprises an analog to digital converter for each shunt and a processor for receiving the digital signals from the converters and determining whether a current imbalance exists. In this case, the temperature sensor may be built into (i.e. integrated) into the analog to digital converter. The temperature sensor is preferably an electronic semiconductor temperature sensor mounted directly onto the shunt with a thermally conducting glue.
Each shunt preferably takes the form of a composite strip having conductive portions at its ends and a resistive portion interconnecting the conductive portions. Such composite strips can be mass produced inexpensively to very high tolerances which makes them extremely suitable for this purpose.
The analog to digital converter for each shunt may include a delta-sigma modulator, which generates a high frequency single digital data stream which is converted by decimation filtering to a multibit digital data stream at a lower frequency.
The analog to digital converter for each shunt is preferably connected to the processor through an isolation barrier so that the converter can float at the voltage level of the shunt which it serves. The decimation filtering may be effected entirely in the converter, entirely in the processor or split between the converter and the processor.